This invention relates broadly to methods and tools for measuring formation geomechanical parameters as a function of both depth and azimuth, whereby features of the formation are determined and imaged.
The art of sonic well logging for use in determining formation parameters is a well established art. Sonic well logs are typically derived from sonic tools suspended in a mud-filled borehole by a cable. The tools typically include a sonic source (transmitter) and a plurality of receivers which are spaced apart by several inches or feet. Typically, a sonic signal is transmitted from the transmitter at one longitudinal end of the tool and received by the receivers at the other, and measurements are made every few inches as the tool is drawn up the borehole. The sonic signal from the transmitter or source enters the formation adjacent the borehole, and the arrival times of the compressional (P-wave), shear (S-wave) and Stoneley (tube) waves are detected by the receivers. The receiver responses are typically processed in order to provide a time to depth conversion capability for seismic studies as well as for providing the determinations of formations parameters such as porosity. It has long been known that the drilling of a borehole into a formation disturbs the stress field that was present in the formation prior to the existence of the borehole. The drilling of the borehole results in circumferential and radial stress concentrations around the borehole, where the resulting stress field is strongly anisotropic at the borehole wall, but the effects of the borehole decrease rapidly with distance into the formation. It has also been established that acoustic velocities in rock are sensitive to applied stress, with both compressional and shear velocities increasing with hydrostatic stress. Uniaxial stress produces compressional and shear wave anisotropy and shear wave birefringence (velocity dependent on polarization). These results have been related by A. Nur, xe2x80x9cEffects of Stress on Velocity Anisotropy in Rocks with Cracksxe2x80x9d, Journal Geophysics. Res.; Vol. 76, 8, p. 2022 (1971), and by D. L. Anderson et al., xe2x80x9cThe Effect of Oriented Cracks on Seismic Velocitiesxe2x80x9d, Journal Geophysics Res.; Vol. 82 p.5374 (1974), to stress-induced anisotropy of microcrack orientations. U.S. Pat. No. 5,544,127, issued Aug. 6, 1996, to Winkler, a co-inventor of the present invention, discloses the use of a sonic borehole tool to measure velocity around the borehole as a function of azimuth. In this patent Winkler teaches that formation properties can be determined from a knowledge of velocity as a function of azimuth, and that the azimuthal direction of minimum velocity around the borehole predicts the propagation direction of artifically induced hydrofractures. He further teaches that sonic velocity variation around the borehole at a particular depth of the borehole may be taken as an indication of susceptibility to failure, with higher velocity variations indicative of a more poorly consolidated formation or a formation with a large uniaxial stress. He further teaches that the curvature of the velocity versus stress curve in the formation is indicated by how poorly a sine wave fits to the velocity data. He further teaches that other parameters of the formation may be obtained by fitting a best fit curve to the azimuth versus velocity data, where adjustable parameters of the best fit curve constitute the formation parameters. Compared to alternative techniques using resistivity measurements, the resolution of sonic techniques is poor. Despite the knowledge which has been accumulated over the years regarding stress fields in formations around a borehole, sonic borehole tools having never been used to capture data of sufficiently high resolution for useful velocity imaging of the borehole wall, or to provide real time indications of thin beds, fractures and vugs as a function of depth and azimuth.
The art of resistance measurement well logging for use in determining formation parameters is also a well established art. The use of resistive measurements is discussed in U.S. Pat. No. 5,463,320, issued Oct. 31, 1995, to Bonner et al. Bonner discloses a logging tool for use in determining the resistivity of an underground formation surrounding a borehole. The tool comprises a mandrel with two transmitters spaced apart thereon, each serving to establish a current in the mandrel and in the underground formation. A series of electrodes are spaced along the body between the transmitters and sensors, located at each electrode, measure radial current flow along a path from the mandrel to the underground formation via a respective electrode. Sensors also provide the axial current flowing along the whole mandrel and at positions corresponding to each electrode. A method of determining the formation resistivity includes the steps of measuring the radial currents R1 R2 from the mandrel to the formation via each electrode and obtaining the axial current M01 M02 along the mandrel at each electrode due to each transmitter; measuring the total axial current M12 along the mandrel from the first or second transmitter and deriving the resistivity of the formation from the radial focused current Rc for each electrode according to the relationship Rc=1/M21 (M02R1+M01R2). However, the resistivity technique does not work in the presence of high-resistivity fluids such as oil-based drilling muds. (OBM""s) as often present in LWD operations, or high-resistivity borehole fluid as often present in wireline operations.
It is therefore an object of the invention to provide methods and tools for producing formation velocity image data at a sufficiently high resolution to identify vugs, worm holes, thin beds, dip angles, fractures, breakouts, and rifling (drilling-induced coherent roughness). It is a further object of the invention to provide tools and methods for producing formation velocity image data in the presence of high-resistivity fluids, including oil-based drilling muds (OBM""s).
It is a further object of the invention to produce a velocity image data set so as to provide real time velocity images of formation surrounding a borehole while drilling as well as during open hole logging.
In accordance with the objects of the invention, the invention provides methods and tools for real time velocity imaging of a borehole wall with sufficiently high resolution to identify vugs worm holes, thin beds, dip angles, fractures, breakouts, and rifling (drilling-induced coherent roughness), for both open hole logging and logging while drilling in the presence of OBM""s.
The present invention provides a method for velocity imaging a borehole wall by measuring an ultrasonic velocity in a portion of a borehole wall at a plurality of azimuths and depths to produce a velocity value at each of the plurality of azimuths and depths, and using the velocity values as a two-dimensional image data set. Measuring a velocity value includes dividing receiver spacing by difference of arrival times of an ultrasonic pulse refracted from the borehole wall at first and second spaced-apart receivers.
The present invention provides a method for velocity imaging a borehole wall by transmitting an ultrasonic pulse through the borehole wall, receiving at a receiver an ultrasonic pulse refracted from the borehole wall, and producing velocity image data values indicative of a time of flight of an ultrasonic pulse between transmitter and receiver at a plurality of azimuths and depths to produce a velocity image data set.
A preferred embodiment of the method for producing a velocity image data set according to the present invention includes the steps of: a) transmitting an ultrasonic pulse through the borehole wall; b) receiving at first and second spaced-apart receivers an ultrasonic pulse refracted from the borehole wall; c) producing a velocity image data value indicative of difference of arrival times at first and second spaced-apart receivers; d) repeating steps a)-c) at a plurality of azimuths to produce a velocity image data value at each of the plurality of azimuths; and e) repeating steps a)-d) at a plurality of depths to produce an image data set.
Step (c) includes producing a velocity image data value by dividing receiver spacing by difference of arrival times. It also includes determining a compressional pulse arrival time by digitizing a signal from a receiver, determining noise level, setting a compressional amplitude threshold to be a multiple of the noise level, determining compressional amplitude threshold time as time at which an absolute value of the signal exceeds the threshold for compressional pulse, picking the first zero-crossing after threshold time, and setting compressional pulse arrival time equal to the time of first zero-crossing. It also includes the step of determining a pseudo-Raleigh pulse arrival time by setting a pseudo-Raleigh amplitude threshold to be a multiple of the larger of the first two extrema of signal following the first zero-crossing, determining pseudo-Raleigh amplitude threshold time as time at which an absolute value of the signal exceeds the threshold for pseudo-Raleigh pulse, picking the first zero-crossing after pseudo-Raleigh threshold time, and setting pseudo-Raleigh pulse arrival time equal to the time of first zero-crossing. It also includes the step of associating the velocity image data value derived from the compressional pulse arrival time with a compressional velocity image data set and associating the velocity image data value derived from the pseudo-Raleigh pulse arrival with a pseudo-Raleigh velocity image data set. It also includes the step of calculating a correction factor to correct for error in effective spacing between receivers according to critical angle and formation velocity.
The present invention provides a tool for producing a velocity image data set representing formation surrounding a borehole. In a basic embodiment, the tool includes an elongate body that is moveable through the borehole; an ultrasonic transmitter attached to the elongate body for transmitting pulses of ultrasonic waves in the frequency range 50-500 kHz into the borehole wall; at least one ultrasonic receiver, attached to the elongate body and spaced apart from the transmitter, for receiving refracted ultrasonic pulses from the borehole wall; and a processor, coupled to the ultrasonic receiver for producing velocity image data values indicative of time of flight of an ultrasonic pulse.
An alternative embodiment uses a single transmitter and a single receiver to produces a velocity image data value indicative of time of flight between transmitter and receiver.
A first preferred embodiment of the tool of the present invention is a wireline tool for real time velocity imaging of a borehole wall. The wireline tool includes an elongated body adapted for suspension from a wireline; an ultrasonic transmitter attached to the elongate body for transmitting ultrasonic pulses into the borehole wall; a plurality of pairs of azimuthally aligned pairs of receivers located at azimuthal intervals around the elongate body, each pair of receivers including first and second spaced-apart receivers; and a processor coupled to receive signals from a pair of receivers; the processor having means for producing a velocity image data value indicative of difference of arrival times at first and second spaced-apart receivers.
The processor further includes means for associating a difference of arrival times with a specific pair of first and second spaced-apart receivers such as to produce velocity image data values as a function of azimuth; means for associating a depth with each difference of arrival times such as to produce velocity image data values as a function of azimuth and depth; means for adding velocity image data values to a stored velocity image data set; means for identifying an arrival as a compressional pulse arrival or as a pseudo-Raleigh pulse arrival; means for determining a difference of arrival times at the first and second spaced-apart receivers; means for dividing spacing between first and second receivers by difference of arrival times; means for first motion detection; and means for first zero crossing detection.
A second preferred embodiment of the tool of the present invention is an LWD tool for real time velocity imaging of a borehole wall while drilling. The LWD tool includes an elongated body which is a portion of drill string proximate to a drill bit; an ultrasonic transmitter attached to the elongate body for transmitting ultrasonic pulses into the borehole wall; a pair of first and second spaced-apart receivers attached to the elongate body for receiving refracted ultrasonic pulses from the borehole wall; and a processor coupled to the ultrasonic receiver for producing velocity image data values indicative of time difference of arrival times at first and second spaced-apart receivers.
The second preferred embodiment, the LWD tool, further comprises a mud pulse data transmitter for transmitting velocity image data values to the surface station; a transmitter mount for attaching the transmitter to the drill string; and a receiver mount for attaching the pair of receivers to the drill string. The receiver mount is sized to minimize standoff of the pair of receivers from the borehole wall.
The processor of the second preferred embodiment includes means for associating a difference of arrival times with an azimuthal position of first and second spaced-apart receivers such as to produce a series of velocity image data values with an azimuthal position attribute as the drill string rotates in the borehole; means for associating a difference of arrival times with a depth such as to produce a velocity image data value with an azimuthal position attribute and a depth attribute; means for adding velocity image data values to a stored velocity image data set; means for identifying an arrival as a compressional pulse arrival; means for first motion detection; means for identifying an arrival as a pseudo-Raleigh pulse arrival; means for first zero crossing detection; means for determining a difference of arrival times at the first and second spaced-apart receivers; and means for dividing receiver spacing by difference of arrival times at the first and second spaced-apart receivers.
The receiver of the second preferred embodiment is a directionally sensitive receiver oriented at 25 degrees to the drill string axis.
The transmitter of the second preferred embodiment is a directional transducer for generating directional pulses of ultrasonic waves in the frequency range 50-500 kHz. The transmitter is oriented at an angle to the drill string axis such that ultrasonic compressional waves propagate toward the receiver along a portion of the borehole wall longitudinally with respect to the borehole. The angle is selected to optimize for compressional waves.